Chapter 17
Cytoskeleton
Essential
Cell Biology
FOURTH EDITION
Copyright © Garland Science 2014
Alberts • Bray • Hopkin • Johnson • Lewis • Raff • Roberts • Walter
國
立
臺
灣
海
洋
大
學
生
命
科
學
暨
生
物
科
技
學
系
教
師
:
陳
秀
儀
2462-2192 ext 5519
sherry0930@mail.ntou.edu.tw
Page 566
Page 566
-microtubules are hollow cylinders with an outer diameter of
about 25 nm and an inner diameter of about 15 nm.
-
The wall of the microtubule consists of longitudinal arrays
of protofilaments, usually 13 of them arranged side by side
around the hollow center, called the lumen.
-
Each protofilament is a linear polymer of tubulin. Tubulin is a
dimeric protein consisting of two similar but distinct
polypeptide subunits, a-tubulin and b-tubulin.
Microtubules
-They have a diameter of about 7 nm, which makes them the
smallest of the major cytoskeletal components.
-Microfilaments are polymers of the protein actin. Actin is
synthesized as a monomer called
G-actin (G for globular).
-
G-actin monomers polymerize into long strands of F-actin (F for
filamentous), with each strand about 4 nm wide.
-
Each microfilament consists of a chain of actin monomers that are
assembled into a filament with a helical appearance and a diameter
of about 7 nm
Microfilaments
-Intermediate filaments have a diameter of about 8–12 nm,
larger than the diameter of microfilaments but smaller than
that of microtubules.
-
The basic structural unit is a dimer of two intertwined,
intermediate filament polypeptides. Two such dimers align
laterally to form a tetrameric protofilament. Protofilaments
then interact with each other to form an intermediate
filament that is thought to be eight protofilaments thick at
any point, with protofilaments probably joined end to end
in an overlapping manner.
Intermediate filaments
Bacteria Have Cytoskeletal Systems That Are Structurally
Similar to Those in Eukaryotes
(actin-like)
(tubulin-like)
(intermediate filament-like)
a gram-positive, round-
shaped bacterium
a Gram-negative,
oligotrophic bacterium
INTERMEDIATE FILAMENTS
INTERMEDIATE FILAMENTS
•
Intermediate Filaments Are Strong and
Ropelike
•
Intermediate Filaments Strengthen Cells
Against Mechanical Stress
•
The Nuclear Envelope Is Supported by a
Meshwork of Intermediate Filaments
Figure 17–3
Intermediate filaments form a strong, durable
network in the cytoplasm of the cell.
Page 567
Pore complexes (TEM). Each pore is ringed
by protein particles.
Nuclear lamina (TEM). The netlike lamina
lines the inner surface of the nuclear envelope.
Nucleus
Nucleus
Nucleolus
Chromatin
Nuclear envelope:
Inner membrane
Outer membrane
Nuclear pore
Rough ER
Pore
complex
Surface of nuclear envelope.
TEM of a specimen prepared by
a
special technique known as
f
reeze-fracture
.
Close-up of nuclear
envelope
Ribosome
1 µm
1 µm
0.25 µm
The Nucleus
細
胞
核
Page 567
© 2005 Elsevier
Page 567
Page 567
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Desmosome structure
They are
especially
abundant in
skin, heart
muscle, and
the neck of
the uterus
Page 567
Figure 17–4
Intermediate filaments are like ropes made of
long, twisted strands of protein.
Page 568
Page 568
Structural Similarities of Intermediate Filament Protein
310-318 a.a
.
Page 568
Page 569
Figure 17–5
Intermediate filaments are divided into four major classes.
These classes can include numerous subtypes. Humans, for example,
have more than
50 keratin genes.
P
disassembly
*IFs differ greatly in amino acid composition from tissue to
tissue (
intermediate filament typing)
(connective tissue)
參
考
用
A mutant form of keratin makes skin more
prone to blistering
normal skin
skin from mutant mouse
The importance of this function is illustrated by the rare human genetic disease
epidermolysis bullosa simplex
, in which mutations in the keratin genes
interfere with the formation of keratin filaments
in the epidermis.
Page 569 & 570
the
plectin (
green
)
links an
intermediate filament (
blue
)
to
three microtubules (
red
)
.
The
yellow dots
are gold particles
linked to antibodies
that recognize plectin
Page 570
Many of the intermediate filaments are further stabilized
and reinforced by accessory proteins, such as
plectin
Intermediate filaments support and strengthen the
nuclear envelope
Page 570 & 571
The
disassembly
and
reassembly
of the nuclear lamina are
controlled by the
phosphorylation
and
dephosphorylation
of
the lamins.
Page 570 & 571
Children with
progeria
have wrinkled skin, lose their teeth
and hair, and often develop severe cardiovascular disease
by the time they reach
their teens.
Figure 17–9
Defects in a
nuclear lamin can
cause a rare
class of
premature aging
disorders called
progeria
Page 571
1.Which
of
the
following
types
of
cells
would
you
expect
to
contain
a
high
density
of
intermediate
filaments in their cytoplasm
? explain your answers.
a. Amoeba proteus (a free-living amoeba)
b. skin epithelial cell
c. smooth muscle cell in the digestive tract
d. Escherichia coli
e. nerve cell in the spinal cord
f . sperm cell
g. Plant cell
MICROTUBULES
MICROTUBULES
•
Microtubules Are Hollow Tubes with
Structurally Distinct Ends
•
The Centrosome Is the Major Microtubule-
organizing Center in Animal Cells
•
Growing Microtubules Display Dynamic
Instability
•
Dynamic Instability is Driven by GTP
Hydrolysis
•
Microtubule Dynamics Can be Modified by
Drugs
MICROTUBULES
MICROTUBULES
•
Microtubules Organize the Cell Interior
•
Motor Proteins Drive Intracellular Transport
•
Microtubules and Motor Proteins Position
Organelles in the Cytoplasm
•
Cilia and Flagella Contain Stable Microtubules
Moved by Dynein
*Microtubules originate from
microtubule-organizing
centers (MTOC)
within the cell.
Fluorescence
micrograph of a
cytoplasmic array
of microtubules in a
cultured fibroblast
Page 572
Each
γ
-tubulin ring complex
serves as the starting point, or
nucleation site
, for the growth of one microtubule
Page 573
Page 573
Page 573
-tubulin:
--- a minor species of tubulin
--- complexes of
-tubulin
form ring structures
that contain 10-13
-
tubulin molecules
(They serve to nucleate
the assembly of
new microtubules)
墊
圈
Page 573
slow
Assembly = disassembly
fast
Purified αβ-tubulin dimers at a high concentration
can polymerize into microtubules spontaneously
in vitro
Page 574
Figure 17–13
Each microtubule grows and
shrinks independently of its neighbors.
The array of microtubules anchored in a
centrosome is continually changing, as new
microtubules grow (
red arrows
) and old
microtubules shrink (
blue arrows
).
Page 574
The selective stabilization of microtubules can
polarize a cell
nonpolarized
cell
Page 574
Figure 17–15
GTP hydrolysis controls the dynamic instability
of
microtubules.
Dynamic Instability Model:
involves a switch from
growth to shrinkage or from shrinkage to growth of
the microtubule or microfilament
Page 575
GTP
GTP
-Tubulin-GTP
-Tubulin-
GDP
Structure polarity
and
-tubulin
dimer
can
each
bind
one
GTP.
The
GTP
in
-tubulin
is
never
hydrolyzed
and
is
trapped
by
the
interface
between
-
and
-
subunits.
By
contrast,
The
GTP-binding
site
on
-subunit
is
at
the
surface
of
the
dimer,
and
can
be
hydrolyzed,
and
the
resulting
GDP
can be exchanged for free GTP.
Page 575
Page 575
2.Dynamic instability causes microtubules either to grow or to
shrink rapidly. consider an individual microtubule that is in its
shrinking phase.
a.
What must happen at the end of the microtubule in order for
it to stop shrinking and to start growing again?
b.
How would a change in the tubulin concentration affect
this switch?
c.
What would happen if only GDP, but no GTP, were
present in the solution?
d.
What would happen if the solution contained an analog of
GTP that cannot be hydrolyzed
Microtubule Dynamics Can be Modified by Drugs
Page 575 & 576
The inactivation or destruction of the mitotic spindle
eventually kills
dividing cells.
If a cell in mitosis is exposed to then drug
colchicine
, which
binds tightly to free tubulin dimers and prevents their
polymerization into microtubules, the mitotic spindle rapidly
disappears, and the cell stalls in the middle of mitosis, unable
to partition the
chromosomes into two groups.
Figure 17–16
Microtubules guide the
transport of organelles,
vesicles, and macromolecules in both directions along
a nerve
cell axon.
Page 576
Motor Proteins Drive Intracellular Transport
Page 577 & 578
Both kinesins and
dyneins move along microtubules using
their globular heads
Page 577 & 578
Different motor proteins
transport different types of cargo
along
microtubules.
Page 577 & 578
Microtubules help position organelles in a eukaryotic cell
Page 579
Figure
17–20
Microtubules
help
position
organelles
in
a
eukaryotic
cell.
(A)
Schematic
diagram
of
a
cell
showing
the
typical
arrangement
of
cytoplasmic
microtubules
(
dark
green
)
,
endoplasmic
reticulum
(
blue
)
,
and
Golgi
apparatus
(
yellow
).
The
nucleus
is
shown
in
brown,
and
the
centrosome
in
light
green.
(B)
One
part
of
a
cell
in
culture
stained
with
antibodies
to
the
endoplasmic
reticulum
(
blue,
upper
panel
)
and
to
microtubules
(
green,
lower
panel
).
Kinesin
motor
proteins
pull
the
endoplasmic
reticulum
outward
along
the
microtubules.
(C)
A
different
cell
in
culture
stained
with
antibodies
to
the
Golgi
apparatus
(
yellow,
upper
panel
)
and
to
microtubules
(
green,
lower
panel
)
.
In
this
case,
cytoplasmic
dyneins
pull
the
Golgi
apparatus
inward
along
the
microtubules
to
its
position
near
the
centrosome,
which
is
not
visible
but
is
located
on
the
Golgi
side
of
the
nucleus.
(B,
courtesy
of
Mark
Terasaki,
Lan
Bo
Chen,
and
Keigi
Fujiwara;
C, courtesy of Viki Allan and Thomas Kreis.)
Outer microtubule
doublet
(a) A longitudinal section of a cilium shows micro-
tubules running the length of the structure (TEM).
(c) Basal body: The nine outer doublets of a
cilium or
flagellum extend into the basal body,
where
each doublet joins another microtubule
to form a ring of nine triplets. Each triplet is
connected
to the next by non-tubulin proteins
(blue). The
two central microtubules terminate
above the
basal body (TEM).
(b) A cross section through the cilium shows the ”9 + 2“
arrangement of microtubules (TEM). The outer micro-
tubule doublets and the two central microtubules are
held together by cross-linking proteins (purple in art),
including the radial
spokes. The doublets also have
attached motor proteins,
the dynein arms (red in art).
Dynein arms
Central
microtubule
Outer doublet
cross-linking
proteins
Radial
spoke
Microtubules
Plasma
membrane
Basal body
0.5 µm
0.1 µm
0.1 µm
Cross section of
basal body
Triplet
Ultrastructure of a Eukaryotic Flagellum or Cilium
Plasma
membrane
Page 579 & 582
Microtubules in a cilium or flagellum are arranged in a “9 + 2”
array
Page 579, 582 & 583
FIGURE 16-8
Enlarged Views of an Axoneme.
(a)
This micrograph shows an axoneme from a flagellum of
Chlamydomonas
(TEM).
(b)
Diagram of an axoneme in
cross section. The microtubules of the central pair have
13 protofilaments each, as do the A tubules of the outer
doublets. Each B tubule has 11 protofilaments of its own
and shares 5 protofilaments with the A tubule. The dynein
sidearms have ATPase activity and are thought to be
responsible for the sliding of adjacent doublets. The interdoublet
links (nexin connections) join adjacent doublets, and
the radial spokes project inward, terminating near projections
that extend outward from the central pair of MTs.
(c)
Bending
of the axoneme by dynein. Connection of the outer doublet
MTs to the central pair converts sliding of adjacent MTs into
local bending of the axoneme.
In
humans,
hereditary
defects
in
ciliary
dynein
cause
Kartagener’s
syndrome
.
Men
with
this
disorder
are
infertile
because
their
sperm
are
nonmotile,
and
they
have
an
increased
susceptibility
to
bronchial
infections
because
the
cilia
that
line
their
respiratory
tract
are
paralyzed
and
thus
unable
to
clear
bacteria
and debris from the lungs.
Page 583
Page 582,583 & 584
ACTIN FILAMENTS
ACTIN FILAMENTS
•
Actin Filaments Are Thin and Flexible
•
Actin and Tubulin Polymerize by Similar
Mechanisms
•
Many Proteins Bind to Actin and Modify Its
Properties
•
A Cortex Rich in Actin Filaments Underlies the
Plasma Membrane of Most Eukaryotic Cells
•
Cell Crawling Depends on Cortical Actin
•
Actin Associates with Myosin to Form
Contractile Structures
•
Extracellular Signals Can Alter the
Arrangement of Actin Filaments
Actin filaments allow animal
cells to adopt a variety of shapes
and perform a variety of functions
Figure 17–28
Actin filaments allow animal cells to adopt a variety of shapes
and perform a variety of functions. The actin filaments in four different
structures are shown here in
red
: (A) microvilli; (B) contractile bundles in the
cytoplasm; (C) fingerlike
filopodia
protruding from the leading edge of a
moving cell;(D) contractile ring during cell division.
Page 584
Microfilaments and Actin Structures
Page 584
Page 584
Page 585
64
Actin Polymerization
in vitro
Proceeds in Three Phases
Addition of nuclei
accelerates the rate
Page 585
Actin Filament Treadmilling at Steady State
Actin Filament
Treadmilling
at Steady State
Page 586
Treadmilling of actin
filaments and dynamic instability of
microtubules regulate polymer length in different ways
Both microfilament and microtubule
have treadmilling and dynamic instability
Page 585 & 586
Figure 17–31
Treadmilling of actin
filaments and dynamic instability of
microtubules regulate polymer length
in different ways.
(A) Treadmilling occurs
when ATP -actin adds to the plus end of an
actin filament at the same time that ADPactin
is lost from the minus end. When the
rates of addition and loss are equal, the
filament stays the same length—although
individual actin monomers (three of which
are numbered) move through the filament
from the plus to the minus end. (B) In
dynamic instability, GTP-tubulin adds to
the plus end of a growing microtubule.
As discussed earlier, when GTP-tubulin
addition is faster than GTP hydrolysis, a
GTP cap forms at that end; when the rate
of addition slows, the GTP cap is lost,
and the filament experiences catastrophic
shrinkage via the loss of GDP-tubulin from
the same end. The microtubule will shrink
until the GTP cap is regained—or until the
microtubule disappears (see Figure 17–15).
Page 586
Treadmilling
involves a simultaneous gain of
monomers at the plus end of an actin filament and
loss at the minus end: when the rates of addition
and loss are equal, the filament remains the same
size
Page 586